Echo recognition and cancellation systems are adapted for use to reduce acoustic echo in many communication applications. Almost any system having simultaneously-active microphones and speakers can benefit from echo cancellation, including intercoms, public address systems, musical recording and amplification systems, and speakerphones, including speaker modes in cell phones. Reducing noise by eliminating audio, and in some cases electronic, echo improves quality of audio detected by these microphones, prevents disturbing feedback oscillations, and improves intelligibility by those listening to detected audio.
Much noise in a microphone signal arises because the microphone picks up audio signals not just from a person speaking (or other sound source) near the microphone, but also from any transducer such as a loudspeaker that may be located near the microphone; the resulting microphone signal is a superposition of the loudspeaker signal as picked up at the microphone, and signals originating from the sound source. In systems having a first and second interconnected sets of loudspeaker and microphone, such as a full-duplex intercom or speakerphones at each end of a telephone call, not only can the superimposed signal be difficult to understand, but pickup by the second set's microphone of the superimposed signal can lead to oscillation having form of a loud squeal.
Audio echo cancellation is typically done by tapping a speaker drive signal, and delaying and filtering that signal according to a transfer function computed as a best match of a path from loudspeaker to microphone to form a delayed speaker signal, then subtracting this electronically delayed speaker signal from the microphone signal to cancel that portion of the microphone signal that represents audio from the loudspeaker.
The transfer function is not always a perfect match for real echo in a real-world installation. Whenever the transfer function is not perfectly matched, some residual, uncancelled, echo remains in the microphone signal. For example, a prototype speakerphone or cellphone may be analyzed in anechoic chamber to determine a transfer function from its loudspeaker to its microphone, and production phones may then be configured to subtract electronically delayed speaker signals from their microphone signal to improve the microphone signal. While such a device will cancel some echo, such as echoes due to sound paths within the device itself, echoes due to reflection of loudspeaker sounds off room walls and into the microphone will not be cancelled because they were not present when the transfer function was determined are therefore not represented the transfer function; these echoes due to reflection of sounds will remain in the microphone signal as a residual echo.